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Martensitic Stainless Steel
Description of material
VAL2/DS is a micro-resulphured martensitic stainless with a Carbon content capable to warrant a high hardness after heat treatment together with a good machinability and moderate corrosion resistance.
All applications where high hardness with a moderate corrosion resistance are indispensable, such as parts of pumps, pump shafts, gears, wear resistant devices and guides. VAL2/DS, after hardening and tempering, provides a hardness higher than VAL1 and VAL2A. VAL2DS should not be used in the production of sharp blades in professional cutlery manufacturing and in table cutlery, if a better cutting edge of blades is required. In this case, a non-micro resulphured VAL2/DE has to be used.
VAL2/DS has a good resistance to mildly corrosive environments such as fresh water, crude oil, gasoline, alcohol, some beverages and atmospheres. VAL2/DS has its maximum corrosion resistance when in the hardened + low temperature tempered condition and with its maximum hardness. Its use in the annealed condition or any other situation able to reduce the surface hardness and in Chloride containing environments, should be avoided. It should be noted that this grade, as for every kind of stainless steel, surfaces should be free of contaminant and scale, heat tint, and passivated for optimum resistance to corrosion.
In the annealed condition, this grade is suitable for moderate cold forming operations such as cold heading and up-setting.
VAL2/DS has a machinability typical of micro resulphured martensitic grades. Productivity gain depends on the type of machines used, the kind of tools used and their geometry, cutting fluids and kind of machine operations on the pieces produced. VAL2/DS is prone to surface decarburization: a protective atmosphere should be considered in the heat treatment of finished pieces. Grinding and polishing of hardened + tempered material at maximum values of hardness must be carried out with great care in order to avoid the overheating of the surface of the piece resulting in poor corrosion resistance and/or grinding cracks.
This process for martensitic stainless grades is always risky and a special care must be applied in the choice of welding parameters. In any case, if a welding process were required, a preheating is mandatory and the part must be maintained at temperature and followed by immediate annealing or tempering. Fillers of the same composition can be used to obtain mechanical properties close to that of the base metal. Alternatively, austenitic fillers may be used considering an inevitable reduction of these properties. In solid state joining such as Friction Welding, VAL2/DS provides a quality bond line. When friction welded with different grades, a tempering or annealing of the welded piece must be done in order to soften the martensitic structure of HAZ and bond line.
Blooms and ingots require a suitable preheating to avoid cracks and a slow cooling in the furnace after forging. Overheating must always be avoided in order to reduce the risk of internal bursts. An improper cooling could result in stress cooling cracks. Large forgings and large cross – section shapes should be left to cool until their core reach room temperature and, then, immediately, heat treated. A right and suitable heat treatment of pieces after the forging process creates a structure with no or little retained austenite avoiding delayed cracks.
VAL2/DS should be double tempered after hardening in order to reduce or avoid retained austenite obtaining high values of hardness. It’s important to point out that high tempering temperatures impair the corrosion resistance of all martensitic grades.